Bet you typed a number starting with 1.

Thing is, for lower speeds, your download is limited to a multiple of your upload. For an upload speed of 1, the maximum allowed download speed is 3, hence this change. By the time you type the rest of the number for upload speed, the damage is done.

1. use the slider to change the speed
or
2. enable 'limitless download' in preferences>connection>connection. Now the download speed will be recalculated when you type the rest of the number.